approxfun {base} | R Documentation

## Interpolation Functions

### Description

Return a list of points which linearly interpolate given data points, or a function performing the linear (or constant) interpolation.

### Usage

    approx   (x, y = NULL, xout, method="linear", n=50,
              yleft, yright, rule = 1, f=0, ties = mean)

    approxfun(x, y = NULL,       method="linear",
              yleft, yright, rule = 1, f=0, ties = mean)

### Arguments

`x, y` | vectors giving the coordinates of the points to be interpolated. Alternatively a single plotting structure can be specified: see `xy.coords`.
`xout` | an optional set of values specifying where interpolation is to take place.
`method` | specifies the interpolation method to be used. Choices are `"linear"` or `"constant"`.
`n` | If `xout` is not specified, interpolation takes place at `n` equally spaced points spanning the interval [`min(x)`, `max(x)`].
`yleft` | the value to be returned when input `x` values less than `min(x)`. The default is defined by the value of `rule` given below.
`yright` | the value to be returned when input `x` values greater than `max(x)`. The default is defined by the value of `rule` given below.
`rule` | an integer describing how interpolation is to take place outside the interval [`min(x)`, `max(x)`]. If `rule` is `1` then `NA`s are returned for such points and if it is `2`, the value at the closest data extreme is used.
`f` | For `method="constant"` a number between 0 and 1 inclusive, indicating a compromise between left- and right-continuous step functions. If `y0` and `y1` are the values to the left and right of the point then the value is `y0*(1-f)+y1*f` so that `f=0` is right-continuous and `f=1` is left-continuous.
`ties` | Handling of tied `x` values. Either a function with a single vector argument returning a single number result or the string `"ordered"`.

### Details

The inputs can contain missing values which are deleted, so at least two complete `(x, y)` pairs are required. If there are duplicated (tied) `x` values and `ties` is a function it is applied to the `y` values for each distinct `x` value. Useful functions in this context include `mean`, `min`, and `max`. If `ties="ordered"` the `x` values are assumed to be already ordered. The first `y` value will be used for interpolation to the left and the last one for interpolation to the right.

### Value

`approx` returns a list with components `x` and `y`, containing `n` coordinates which interpolate the given data points according to the `method` (and `rule`) desired.
The function `approxfun` returns a function performing (linear or constant) interpolation of the given data points. For a given set of `x` values, this function will return the corresponding interpolated values. This is often more useful than `approx`.

### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

### See Also

`spline` and `splinefun` for spline interpolation.

### Examples

    x <- 1:10
    y <- rnorm(10)
    par(mfrow = c(2,1))
    plot(x, y, main = "approx(.) and approxfun(.)")
    points(approx(x, y), col = 2, pch = "*")
    points(approx(x, y, method = "constant"), col = 4, pch = "*")

    f <- approxfun(x, y)
    curve(f(x), 0, 10, col = "green")
    points(x, y)
    is.function(fc <- approxfun(x, y, method = "const")) # TRUE
    curve(fc(x), 0, 10, col = "darkblue", add = TRUE)

    ## Show treatment of 'ties' :

    x <- c(2,2:4,4,4,5,5,7,7,7)
    y <- c(1:6, 5:4, 3:1)
    approx(x,y, xout=x)$y # warning
    (ay <- approx(x,y, xout=x, ties = "ordered")$y)
    stopifnot(ay == c(2,2,3,6,6,6,4,4,1,1,1))
    approx(x,y, xout=x, ties = min)$y
    approx(x,y, xout=x, ties = max)$y

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